The life cycle of power station components is at present usually determined by initially selecting the assemblies of the power station component that are subject to the highest stress and then individually determining the exhaustion for these assemblies. Methods of creep and cyclic stress exhaustion in particular are used when determining the exhaustion.
Thus, the use of finite element method for a detailed stress analysis of assemblies of power station components is known. With this method, differential equation systems, which describe the stresses present in the assembly, are solved. Input variables for the differential equations are, for example, the pressure and temperature patterns of a medium around the assembly. The material stresses are then determined in precise detail from the calculated stresses.
A disadvantage with the known methods is that they provide only results regarding the exhaustion of individual assemblies or even only of sections of assemblies, whereas an assembly-overlapping, so-called integral, assessment of the life cycle of the complete component is possible, if at all, only at great expense. A higher-level assessment of this kind normally requires the results for the individual assemblies to be interpreted by experts.
Another possible method of determining the life cycle of power station components is the specification, during the design of the power station component, of a pre-defined number of operating hours and starts or trips (i.e. output changes of the power station component).
A comparatively simple, known method is therefore based on counting the operating hours and load changes on the basis of pre-calculated pressure and temperature stresses in the assemblies of a power station component and then comparing these with experimentally determined values of the creep strength and number of insipient crack load changes. This method is used to determine the life cycle of power station boilers (see DIN EN 12952/4:2001-10 “Water tube boilers and system components, Part 4: In-service calculation of the life cycle expectation, German version EN 12952-4:2000”), with such boilers being a particular type of power station component for which a specific type of life cycle determination is necessary due to the particular operating conditions.
The last-named possibility usually fails due to the fact that how the power station component is run in actual operation usually differs from that visualized during design.